Rotor for an axial flux rotating electrical machine compressed with a band

ABSTRACT

Embodiments involve rotors for axial flux induction rotating electric machines that use a soft magnetic composite for the rotor core. A first embodiment is directed to a rotor for a rotating electrical machine that transmits magnetic flux parallel to a shaft of the rotor. The rotor includes a rotor winding and a plurality of cores. The rotor winding consists of a solid piece of conductive material that comprises a plurality of cavities. Each core is placed in a respective cavity and comprises a highly resistive isotropic ferromagnetic powder.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.17/034,503, filed on Sep. 28, 2020, which claims priority to U.S.Provisional Patent Application No. 62/928,282, filed Oct. 30, 2019, U.S.Provisional Patent Application No. 63/011,034, filed Apr. 16, 2020, andU.S. Provisional Patent Application No. 63/046,868, filed Jul. 1, 2020,each of which is incorporated by reference in its entirety.

FIELD OF THE INVENTION

This application generally relates to rotating electrical machines, suchas motors and generators.

BACKGROUND

Rotating electrical machines, such as motors and generators, are knownto exist in various different types and geometries. For example, somerotating electric machines rely on permanent magnets for magnetic polepieces. These permanent magnets, for performance reasons, typicallyinclude rare earth elements such as neodymium, samarium, cerium,terbium, praseodymium, gadolinium, and dysprosium. Permanent magnetsynchronous machines are often desirable because they exhibit a highstarting torque, high efficiency, and high power density.

However, the rare earth metals that high performance permanent rotatingelectrical machines rely on are produced primarily by mines in only afew regions of the world. The rare earth elements used for the magnetsadd considerable cost to the machine, and the process of mining them isnearly always very damaging to the environment. They introducevulnerabilities in the supply chain. Finally, when heat is applied thereis a risk they could lose their magnetic properties.

An alternative to a permanent magnet rotating electrical machine is aninduction rotating electrical machine. Induction machines do not rely onpermanent magnets. Instead, induction machines use induced magneticfields brought about by changing currents to operate. The cores of aninduction machine may simply be iron, or other material that ismagnetically permeable. While induction machines avoid the need for rareearths, widely available induction machines are thought to be unsuitablefor many applications because of their lower power density and lowerstarting torque.

In general, rotating electrical machines exist in two geometries: radialand axial. Radial flux motors transmit magnetic flux perpendicular tothe motor's shaft. With these radial flux motors, the rotor—thecomponent of the motor that rotates—and the stator—the component of themotor that remains stationary—both tend to be cylindrical in shape andconcentric with one another. For example, the stator may enclose therotor and transmit magnetic flux inward toward the rotor. The magneticflux applies torque to the rotor, making it turn. An example is aconventional squirrel cage motor.

In contrast, axial flux motors transmit magnetic flux parallel to themotor shaft. Instead of being concentric cylinders, in an axial fluxmotor the rotor and stator may be discs mounted parallel to one anotherand perpendicular to the motor shaft. The stator applies magnetic fluxthrough the rotor, creating torque. An example of such an axial fluxmotor was disclosed by Pyrhonen et al. in U.S. Pat. Pub. No.2008/0001488, entitled “Axial Flux Induction Electric Machine.”

In general, axial flux motors tend to be more compact than radial fluxmotors having the same power. In other words, an axial flux geometrytends to produce a higher power density. All other things, includingaxial length, being equal, increasing a radius of a radial flux motormay increase power output by the difference in radius squared. Incontrast, all other things being equal, increasing a radius of an axialflux motor may increase power output by the difference in radius cubed.The result is that greater power output can be achieved by using lessmaterials than would be required for a radial machine of equivalentpower.

In electric cars, permanent magnet machines have conventionally beenused due to their high power density, superior starting torque, andcompact size. Permanent magnet motors in general are thought to providegreater starting torque than induction motors because the permanentmagnets are already excited. However, the torque from a permanent magnetmotor at a given excitation voltage is inversely proportional to rotorrotational speed owing to back EMF (electromagnetic flux) induced in thestator coils (e.g., as a consequence of Faraday's Law). Morespecifically, while the efficiency of a permanent magnet (PM) motorinitially increases with rotational velocity (from zero), once it isclose to rated speed, the efficiency drops precipitously owing to thelack of torque (from back EMF) and the increased iron losses (hysteresisand eddy losses). Thus, at high speeds, the efficiency of permanentmagnet machines can fall off dramatically, which can result in wastedelectricity and limited torque. It is the (nearly) linear loss of torquewith velocity, however, that is the major design limitation for PMmotors in traction motor applications such as electric cars. Indeed, itis for this reason that some electric vehicles use a pair of motors: aPM motor for low driving speeds, and an induction motor for high drivingspeeds.

Conventionally, induction machines are thought to have poor startingtorque. While induction machines are typically preferable to PM motorsat higher RPMs, conventionally, an induction machine that gives greaterthan about 40% of full load torque when in a locked rotor state isdifficult to design. This is in part because induction machines tend toperform poorly at high slip.

Eddy currents can be present in any type of rotating electrical machine,but are conventionally thought to be particularly problematic in axialmachines. Eddy currents are electrical currents that circulate aroundinside conductive materials in a manner similar to swirling eddies in ariver. They generate unwanted losses (and thus heat) in the systems,particularly in high frequency applications. For example, some eddycurrents may be currents that are induced in the metal core materialitself by the changing magnetic field as alternating current produces achanging flux. In addition, eddy currents may be caused in largeconductors by interactions with other conductors and current loops inthe rotating machine.

To reduce eddy currents, lamination stacks are used in the rotor andstator assemblies. Lamination stacks include a plurality of thin steellayers and insulation between them that increases resistivity of themagnetic material in a direction that is perpendicular to the insulationwhile maintaining magnetic properties of the laminated material in otherdirections. The laminated material may be for example iron or apermanent magnet material. Such lamination stacks can be used either inthe rotor disk, such as in the rotor cores, or as the stator core.However, the lamination stack reduces the density of material,deteriorating the magnetic properties desired. They also can create apoint of failure that reduces the machine's durability and reliabilitydue to stress and fatigue.

For example, motors in electric scooters and bicycles may experiencemomentary high acceleration forces when these vehicles hit a streetobstacle, such as a pothole. These high acceleration (“gee”) forces cancause the weld that holds laminations together to experience fatigue oreven brittle failure if the acceleration and jerk are sufficientlylarge. Should the lamination weld(s) fail, the motor itself would ceaseto function. This can create a severe safety hazard.

Recently, soft magnetic composite (SMC) materials have become available.These soft magnetic composites are mixtures of ferromagnetic powderparticles coated with an electrically insulating layer (typically asteam-formed oxide) and nonmetallic binders (such as phosphorus). Thesecomposites can be formed into complex geometric shapes. The result is anelectrically resistive, ferromagnetic material with isotropicproperties. The isotropic properties allow it to carry magnetic flux inall directions inside a material, unlike a laminated stack. And, SMC'shigh resistivity enables the designer of a magnetic system to moreprecisely direct the flow of current within a magnetic system whencompared to using a lower resistivity solution such as laminationstacks. An example of such a composite material is the SOMALOY compositepowder available from Höganäs AB of Höganäs, Sweden.

However, these soft magnetic powders cannot be machined adequately. Thatis, they are difficult to machine with conventional methods, and cannotbe machined at all by electric discharge methods (wire and die-sinkEDM). They universally have very low tensile strength, impact strength,and bending modulus. When subjected to large amounts of compressiveforce, they have a tendency to convert from a consolidated form backinto powder, or to chip or fragment. Thus, while these magneticmaterials have been used in stators for electric motors, their utilityhas been limited.

Often, stators have teeth with wires, called windings, wrapped aroundthem. The windings may be insulated from one another, allowing currentto flow only along the wires. Such insulation reduces the density of theconductive materials, and ultimately can limit the power density of therotating electric machine. Conventionally, stator teeth have, at theirtop, a tip that closes in toward adjacent teeth, making the teeth widerat the top and narrower at the bottom and making the stator slotspartially closed. This is thought to provide special magnetic reluctanceto the air gap. The tip is thought to allow the magnetic flux to betransmitted in a cleaner sine wave. Any harmonics away from that sinewave can cause additional losses in the rotor.

Consistent with partially closed slots, windings for axial inductionmachines are typically wound by hand. Doing so introduces a number ofproblems. First, it is a labor-intensive process that accounts for asignificant portion of machine costs. Second, it introduces anopportunity for error. Third, the number and type of windings that canbe fixed into or made within the interior annulus between the stator'steeth is limited by the operator's skill and the geometry of the wire,the wire's insulation, and the interior of the stator. Kinks can occurduring the winding, increasing unwanted electrical resistance anddramatically increasing the risk of mechanical and thermomechanicalfatigue failure in the winding. This is especially true for axialstators, because the endturns needed for the coils have a smallerbending radius. The fill factor of the stator's slots is also reduced,thus restricting the amount of H field (total magnetization) that can begenerated in the slot. This limits the power density of the resultingmachine.

Conventionally, air-cooled motors are thought to, in general, have lowerperformance than liquid-cooled motors, because air-cooled motors moreeasily overheat and fail. For that reason, most rotating electricalmachines used to drive electric vehicles are liquid-cooled.Liquid-cooled motors have disadvantages in that they are morecomplicated and, in general, more expensive and less durable thanair-cooled motors.

Rotating electrical machines that produce better efficiency, powerdensity, cost-effectiveness, and durability relative to existingmachines are needed.

SUMMARY

Embodiments involve rotors for axial flux induction rotating electricmachines that use a soft magnetic composite (SMC) for the rotor core. Afirst embodiment is directed to a rotor for a rotating electricalmachine that transmits magnetic flux parallel to a shaft of the rotor.The rotor includes a rotor winding and a plurality of cores arrayedwithin the rotor. The rotor winding consists of a solid piece ofconductive material that comprises a plurality of cavities. Each core isplaced in a respective cavity and comprises a highly resistive isotropicferromagnetic SMC powder.

In a second embodiment, a rotor for an axial flux induction rotatingelectrical machine includes a rotor winding, a plurality of cores, and aplurality of supports. The rotor winding includes a plurality ofcavities and has a first coefficient of expansion. Each core in theplurality of cores is inserted within a respective cavity from theplurality of cavities. Each core includes ferromagnetic powder andrespective grains of the ferromagnetic powder are insulated from oneanother. Finally, each support from the plurality of supports attachesto a respective core from the plurality of cores to a respective cavityfrom the plurality of cavities such that the support remains attached tothe respective core and the respective cavity at a given operatingtemperature range of the rotor given the different first and secondcoefficients of expansion.

In a third embodiment, a rotor for an axial flux rotating electricalmachine includes a plurality of cores, a rotor winding, and a band. Therotor winding consists of a solid disc of conductive material thatcomprises a plurality of cavities. Each of the plurality of cores islocated in a respective cavity of the plurality of cavities. The bandengages an outer edge of the rotor winding and applies compression tothe rotor winding.

In a fourth embodiment, a coil for a rotating electrical machine thattransmits magnetic flux parallel to a shaft of the machine ismanufactured in at least two steps. First, a wire is repeatedly bent atsubstantially 180 degree angles to stack the bent wire in an axialdirection along the shaft of the rotating electrical machine. The wireis bent such that a number of bends in the wire corresponds to a numberof turns of a winding for a stator of the rotating electrical machine.Second, the wire is pressed in a direction of rotation of the rotatingelectrical machine. The wire is pressed with a die having interlacedteeth with a shape to form the wire into the coil to fit over at leastone tooth of a stator of the rotating electrical machine.

In a fifth embodiment, a stator for an axial flux rotating electricalmachine includes a winding and a stator core. The winding is configuredto produce a magnetic field and includes a plurality of coils lapped oneatop another. The stator core includes a base portion and a plurality ofteeth. The base portion shorts magnetic flux. And each tooth transmitsthe magnetic flux and separated by a slot and open such that duringmanufacture the winding is able to be slipped on the plurality of teeth.

In a sixth embodiment, an axial flux rotating electrical machineincludes an endbell, a stator core and a bracket. The stator core ismade of a soft magnetic composite (SMC) and includes a plurality ofteeth, a base portion, and a lip. Each tooth transmits flux from amagnetic circuit that is separated from one another by a slot. The baseportion connects the plurality of teeth and shorts the magnetic circuit.The lip extends from the base portion. Finally, the bracket attaches tothe endbell and engages with the lip to hold stationary the single-piecestator in the endbell.

Method, device, and product-by-process claims are also disclosed.

Further embodiments, features, and advantages of the invention, as wellas the structure and operation of the various embodiments, are describedin detail below with reference to accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated herein and form partof the specification, illustrate the present disclosure and, togetherwith the description, further serve to explain the principles of thedisclosure and to enable a person skilled in the relevant art to makeand use the disclosure.

FIG. 1 shows an assembly for an axial flux machine according to someembodiments.

FIG. 2 shows a method of making a rotor for an axial flux machineaccording to some embodiments.

FIG. 3 shows a core for a rotor for an axial flux induction machineaccording to some embodiments.

FIG. 4 is a diagram that shows how cores are suspended by supports in arotor winding according to some embodiments.

FIGS. 5A-D are diagrams illustrating different ways to suspend cores ina rotor winding according to some embodiments.

FIG. 6 illustrates how a rotor winding is inserted into a band accordingsome embodiments.

FIG. 7 illustrates a stator including a stator core and stator windingsaccording some embodiments.

FIGS. 8A-B illustrate how a coil for stator windings according someembodiments.

FIG. 9 illustrates a method for manufacturing a coil for a statorwinding according to some embodiments.

FIGS. 10A-K are diagrams illustrating an example operation for themethod for manufacturing the coil.

FIG. 11 illustrates how coils are interlaced, interconnected, andinserted on an open stator.

FIGS. 12A-D illustrate two coils lapped on one another withinterconnects welded on to joints.

FIG. 13 illustrates example signals applied to three phases of thestator coils.

FIG. 14 illustrates a stator core manufactured from soft magneticcomposite according to some embodiments.

FIG. 15 is a cross-section illustrating how the stator core is mountedonto an endbell of a rotating electrical machine according someembodiments.

FIGS. 16A-D show in greater detail how a stator can be mounted to anendbell according to embodiments.

FIGS. 17 and 18A-B show how a rotor can be mounted to a shaft accordingsome embodiments.

FIG. 19 shows a rotor for an axial flux machine according to someembodiments.

FIGS. 20-21 show an example axial flux motor according to someembodiments.

FIGS. 22A-D illustrate how an axial flux induction motor produces torqueaccording to some embodiments.

FIGS. 23A-B show a magnetic field and an electric current in a rotor foran axial flux motor according to some embodiments.

FIGS. 24A-B show a cross-section of a rotating electrical machine withan impeller attached to a shaft for cooling according to someembodiments.

FIG. 25 illustrates a rotor for a rotating electrical machine that hasfins for cooling according to some embodiments.

FIG. 26 illustrates a rotor with a cooling fin assembly, according tosome embodiments.

The drawing in which an element first appears is typically indicated bythe leftmost digit or digits in the corresponding reference number. Inthe drawings, like reference numbers may indicate identical orfunctionally similar elements.

DETAILED DESCRIPTION

Disclosed herein is an axial flux induction rotating electrical machinethat solves some of the shortcomings discussed above. For example, arotor uses an insulated ferromagnetic powder for its cores. This allowsfor an axial induction machine that controls eddy currents while beingcheaper, easier and faster to make, and possibly more durable than acomparable machine that uses lamination stacks. Also, being an axialmachine, the machine provides good power density and efficiency, and, byavoiding permanent magnets, the machine provides improvedcost-effectiveness and lessens the environmental impact of itsproduction.

According to some embodiments, the rotor winding is a solid disc ofconductive metal with cavities for the respective high permeabilitycores. In many applications, it is advantageous for the rotor to spin ata high velocity and therefore it must be able to withstand highcentrifugal acceleration forces. The core within the rotor must be fitvery tightly in the surrounding rotor winding to prevent motion that canlead to fatigue failure. As mentioned above, these ferromagnetic powdersmay be extremely brittle and difficult to machine because machining alsoalters the electrical integrity of the insulated powder. Thus, it may bedifficult or impossible to machine or press a core made of theseferromagnetic powders to the adequate tolerances needed to maintain atight enough fit so that the core remains secure in the rotor winding.This is particularly true given the fact that the core and the windingshave different coefficients of expansion and thus expand or contractdifferently when exposed to temperature variations. For these reasons,an interference fit between a core and a rotor winding may beimpossible.

To deal with this issue, embodiments disclosed herein surround at leasta portion of the core with one or more supports made of a material thatis rigid enough to hold the cores and at the same time accommodate thedifferent coefficients of expansion for the cores and winding. Thus, thematerial must be flexible enough to adapt to the different manufacturingtolerances of the cores and winding. The material may be applied toattach the rotor winding with a core's inner portion, that is, a portionthat is closer to a shaft, and the core's outer portion, that is, aportion that is closer to a perimeter of the rotor. On side portions, anair-gap may be allowed. In one example, the shims may be made of avulcanized elastomer. This and other examples are described in greaterdetail below.

As mentioned above, the rotor needs to withstand significant centrifugalforces. A prior approach in an axial machine was disclosed by Pyrhonenet al. in U.S. Pat. Pub. No. 2008/0001488. To withstand these largecentrifugal acceleration forces, Pyrhonen taught to mount the rotorwinding onto a carbon fiber frame. This approach has significantdownsides in terms of cost and difficulty of manufacture.

Embodiments disclosed herein, on the other hand, use a band to compressthe rotor winding and counteract centrifugal forces, maintaining africtional bond between the rotor winding and the respective cores. Theband is made of a strong material, such as a maraging steel, and tightlysurrounds the rotor winding disk, applying a compressive force thatcounteracts centrifugal forces. In this way, embodiments providecost-effective ways of having a single disc of conductive metal as thewinding while maintaining durability and allowing for higher rotationalvelocity.

As mentioned above, conventionally, windings for axial flux inductionmotors are wound by hand. Embodiments disclosed herein provide anautomated way to manufacture coils for a stator and apply the coils ontothe stator. According to embodiments, rectangular wire (also known asbar wire) is de-coiled and straightened. The rectangular wire is thenrepeatedly bent to stack the bent wire in an axial direction along theshaft of the rotating electrical machine. A number of bends in the wirecorresponds to a number of turns of the coils for the rotatingelectrical machine. Then, in a direction of rotation of the rotatingelectrical machine, the wire is pressed with a die having interlacedteeth. In this way, the wire is formed into a shape to fit over at leastone tooth of the stator, resulting in a coil. The coils can be lappedinto one another, interleaved, and interconnected into phases to makethe stator windings.

Moreover, according to an embodiment, coils are fabricated individuallysuch that they can intermesh with one another upon assembly into coilgroups. In this way, after fabrication, they can be assembled into awiring apparatus that can be slipped onto an open stator, reducing laborin the manufacturing process and improving the slots' fill factor.

As mentioned before, one reason why stator windings are conventionallyinserted by hand is that their slots are partially closed, resulting ina widening of the stator teeth at the top. Hence, according to anembodiment, the stator teeth are open in such a way that the assembledstator windings can be slipped directly down onto the stator, obviatingany need to hand-wind the coil assembly.

Cost-effectiveness of the machine is provided for by avoiding the needfor both expensive rare earth materials and by lowering wire insertioncosts. Moreover, having equivalent or greater power density toconventional rare earth machines in a radial configuration results inlower mass. The lower mass results in knock on effects which provideadditional cost savings for applications such as electric cars.

As mentioned above, most motors for electric cars are liquid-cooled.This is expensive and may introduce additional components, not found inair-cooled motors, that can have reliability issues. Some embodimentsmount an impeller to the shaft to drive air through the air gap betweenthe rotor and stator, whisking heat out of the rotor and cooling it.Alternatively or additionally, the rotor may have fins to enhanceconduction and convection of heat.

Reference will now be made in detail to representative embodimentsillustrated in the accompanying drawings. First, assembly of an exampledevice is described with reference to FIG. 1. Second, a rotor and itsfabrication is described in greater detail with reference to FIGS. 2-4,5A-D, and 6. Third, a stator and its fabrication is described in greaterdetail with reference to FIGS. 7, 8A-B, 9, 10A-K, 11, 12A-D, and 13.Fourth, how a stator may be mounted to an endbell is described withrespect to FIGS. 14, 15, and 16A-D. Fifth, how a rotor may be mounted toan axel is described with respect to FIGS. 17 and 18A-B. Sixth, variousalternative embodiments are described with respect to FIGS. 19-21.Seventh, operation of an axial flux induction machine is described withrespect to FIGS. 22A-D and 23A-B. Eighth and finally, FIGS. 24A-B andFIG. 25 illustrate techniques for cooling the axial flux inductionmachine. It should be understood that the following descriptions are notintended to limit the embodiments to one preferred embodiment. To thecontrary, it is intended to cover as many alternatives, modifications,and equivalents as can be included within the spirit and scope of thedescribed embodiments as defined by the claims.

Assembly of an Axial Flux Induction Machine

FIG. 1 shows an assembly for an axial flux machine 100 according to someembodiments. Machine 100 includes two endbells 102A and 102B, twostators 132A and 132B, two bearing assemblies 136A and 136B, shaft 140,and a rotor 134. In this embodiment, rotor 134 is located betweenstators 132A and 132B. This is one configuration; other embodimentsutilizing these components will be described below with respect to FIGS.20 and 21. Each of these components is described in turn.

Endbells 102A and 102B form a housing that encloses machine 100.Endbells 102A and 102B provide support for bearing assemblies 136A and136B and provide protection for the internal components of machine 100,including stators 132A and 132B and rotor 134. Endbells 102A and 102Bmay be mounted to the device utilizing the machine, such as, forexample, a vehicle, turbine, or any industrial device that needs toconvert electricity to torque, or vice versa.

Endbells 102A and 102B are preferably made of a non-ferromagnetic(preferably diamagnetic), electrically resistive material such asstainless steel 310 or 304 or titanium (whether alloy or commerciallypure). Other structurally appropriate non-magnetic materials may be usedas well such as polyether ether ketone (PEEK), polyethylenimine (PEI),or a carbon fiber-, glass fiber-, or aramid fiber-resin composite (wherethe resin could be either a thermoset or thermoplastic).

Endbells 102A and 102B have holes, illustrated with hole 130, down theircenter axis for machine 100's shaft 140 to pass through. Endbell 102Balso has a hole 138 for wiring connecting to stators 132A and 132B topass through. In embodiments, endbells 102A and B may also have holesfor cooling the machine, such as for transmission of gases or othercooling substances. A skilled artisan would recognize various methods inthe art used to circulate air or other substances through machine 100for cooling. The other substances may be forced through machine 100,perhaps using a fan or other means. In this way, machine 100 may be moreactively cooled.

In an embodiment, the space between rotor 134 and stators 132A and B maybe between 0.1-2 millimeters. This embodiment may have an advantage ofimproved airflow/gas flow, which allows heat to be removed from therotor disk, stator, and windings more effectively.

In another embodiment, endbells 102A and B may be sealed, and perhapspressurized. For example, endbells 102A and B may include hydrogen gas.Hydrogen gas is an effective conductor of heat; thus it may helpevacuate heat from components of machine 100. In this embodiment,bearing assemblies 136A and 136B may be airtight. In this embodiment,holes may be used to circulate the gas to an internal or external heatexchanger. To circulate the gas, an internal fan, screw, blower, orregenerative turbine may be used.

Within endbells 102A and B are stators 132A and B and a rotor 134. Thisis one embodiment; others will be described below with respect to FIGS.19-21. Stators 132A and B each include a respective stator core 112A and112B and a respective stator winding 116A and 116B. Stator cores 112Aand 112B preferably are made of a magnetically permeable, highlyresistive material such as SMC powder or silicon steel. In one example,stator cores 112A and 112B may be made of a strip of laminatedelectrical steel wound into a bobbin and cut to form the appropriateshape.

Stator windings 116A and 116B are preferably made of an electricallyconductive material. For example, stator windings 116A and 116B may bemade of copper, aluminum, silver, gold, or other high conductivityelectrical materials. Current is passed through stator windings 116A and116B to induce a magnetic field in stator cores 112A and 112B. Stators132A and B and how they may be fabricated are described in greaterdetail below with respect to FIGS. 7-13.

Stators 132A and B may be mounted in a fixed manner to the respectiveendbells 102A and 102B. If stator cores 112A and 112B are made ofmaterial with appreciable tensile strength, such as silicon steel, holesmay be drilled in stator cores 112A and 112B to attach stator cores 112Aand 112B into the correct position in respective endbells 102A and 102B.

If stator cores 112A and 112B are made of material that does not haveappreciable tensile strength, such as an SMC, other methods of mountingstators 132A and 132B to endbells 102A and 102B may be employed. Onesuch method is described below with respect to FIGS. 14, 15, and 16A-D.

Also mounted to endbells 102A and 102B are bearing assemblies 136A andB. Bearing assemblies 136A and B provide a rigid support for shaft 140while allowing shaft 140 to spin with a minimal amount of friction. Anynumber of common techniques may be used to attach bearing assemblies136A and B to shaft 140 and to the respective endbells 102A and B.Bearing assemblies 136A and B may include a spring to effect a preloadand centering that reacts against the inner axial wall of the endbell(as shown with spring 104A and outer portion 107) and an inner portionthat attaches to shaft in a fixed manner (as shown with inner portion118B). The inner portions may include a bore that supports bearings 110Aand B and allows them to spin with minimal friction. In differentembodiments, bearings 110A and B may be ball bearings or rollerbearings. In this way, bearing assemblies 136A and B allow shaft 140 tospin while providing adequate support.

Shaft 140 is attached to, and abuts perpendicular from, rotor 134. Moredetail on how bearing assemblies 136A and B support shaft 140 and howshaft 140 is attached to rotor 134 is provided below with respect toFIGS. 17 and 18A-B.

Rotor 134 includes a plurality of cores, such as core 106, a rotorwinding 114, and a band 108. Each core is placed within a cavity ofrotor winding 114. The rotor winding 114 is surrounded by band 108.Rotor winding 114 carries the current induced by stators 132A and B. Inan embodiment, rotor winding 114 consists of a solid disc of conductivematerial that comprises a plurality of cavities. The disc may be aflattened cylinder in shape. The rotor and its fabrication are describedin greater detail with reference to FIGS. 2-4, 5A-D, and 6.

In addition, machine 100 may include a position sensor (not shown). Theposition sensor may be able to detect the position, direction ofrotation, and/or the rotational velocity of rotor 134 and/or shaft 140.The position sensor may be mounted either within the interior or uponthe exterior of endbells 102A-B. Data collected from the position sensormay be used to control the phase and amplitude of current applied tostator windings 116A-B.

Rotor for an Axial Flux Induction Machine

FIG. 2 shows a method 200 of making a rotor for an axial flux machineaccording to some embodiments. Method 200 starts at a step 202 withfabrication of a plurality of cores, a rotor winding, and a band. Eachof these components may be fabricated separately as described below.

Each of the plurality of cores, such as core 106 in FIG. 1, may be madeof an electrically resistive, isotropic ferromagnetic powder such as asoft magnetic composite material, for example, the SOMALOY 700 3P SMCpowder. Respective grains of the isotropic ferromagnetic powder may beiron coated with an insulating layer. The grains may be insulated fromone another with a coating, e.g., magnetite, silica, or other insulatingoxide. An example of such a powder is the SOMALOY composite powderavailable from Höganäs AB of Höganäs, Sweden. In this way, embodimentsavoid laminating material while controlling eddy currents.

Core 106 may not be a permanent magnet, but may have to be excited totransmit a magnetic flux. In embodiments, core 106 has a saturationmagnetic flux density greater than 1.5 T or even 2.0 T. Core 106 hasmagnetic flux density of at least 1.1 T, or even at least 1.5 T, whenthe core is subjected to a magnetic field of 4,000 Amps/m. Core 106 mayhave a magnetic permeability of at least 1, 1.5, or even 2. Core 106 mayhave a thermal conductivity of less than 40 W/m*K. Within core 106, theferromagnetic powder may have a density between 7.25 g/cm³ and 7.60g/cm³. Alternatively, the ferromagnetic powder may have a densitybetween 7.49 g/cm³ and 7.58 g/cm³.

To achieve such density, the SMC powder may need to be compressed in adie as illustrated in a diagram 300 in FIG. 3. Diagram 300 illustrates adie 302 having a mold 304 in the shape of core 106. The powder may beplaced in mold 304 and compressed with between 550 and 800 Megapascals(or 39.9 to 58.0 short tons per square inch) of compaction pressure. Thepowder may be repeatedly inserted and compressed until it reachessufficient density. After the powder is compressed into a core, the coreis heated to 900° C. in a nitrogen atmosphere to evaporate the die lubeand to activate the phosphorus binding agent and give the coremechanical strength. Subsequently, the core may be heated tosubstantially 600° C. in a steam atmosphere. These various treatmentsmay cause the powder to bond together into a solid mass (specifically asol colloid).

In other embodiments, core 106 may be made of lamination stacks, acoiling made of ferromagnetic material, and/or soft bulk ferromagneticcores, or some combination thereof. A lamination stack may be a stack ofsheets laminated from one another to promote resistivity. A coiling madeof ferromagnetic material may be a stack that is wrapped around acentral axis in such a way as to laminate each layer within it from oneanother. An example of a soft bulk ferromagnetic core is VanadiumPermendur (also known as Hiperco 50). Vanadium Permendur is aniron-cobalt-vanadium ferromagnetic alloy.

Returning to FIG. 2, a rotor winding is also fabricated in step 202. Asdescribed above, the rotor winding may consist of a solid disc ofconductive material. The disc may be a flattened cylinder in shape.

One example rotor winding 114 is illustrated in FIGS. 5A-B. As shown inthose figures, rotor winding 114 comprises a plurality of cavities 544A. . . N. Between each of the cavities, rotor winding 114 includes rotorbars, such as rotor bar 507. A current is induced in the rotor bars inresponse to the time and spatially varying magnetic field flux that isapplied by the stator axially across the airgap and into the rotor. Thisinduced current in the rotor bars itself produces a magnetic flux thatopposes (but is less than enough to balance) the applied magnetic fluxpassing through the rotor. The induced current in the rotor barsproduces Lorentz force reactions in response to the net magnetic fieldand thereby generates torque. The rotor bars are configured to be narrowenough to cleanly transmit the induced current, avoiding eddy currentsto the greatest extent possible, but wide enough to carry the inducedcurrent without too much resistive losses.

Connecting the rotor bars are two shorting rings: a shorting ring 508and shorting ring 510. Shorting ring 508 and shorting ring 510 are ringsthat short the respective rotor bars. Shorting ring 508 is in theinterior portion of rotor winding 114, and shorting ring 510 is in theexterior portion of rotor winding 114.

As described above, rotor winding 114 may be a single piece ofconductive metal. Thus, shorting ring 508, shorting ring 510, and therotor bars 507 may be different regions of the same piece of metal. Inone embodiment, rotor winding 114 may be made of a chrome-copper alloy,which offers technical advantages in cost-effectiveness, conductivity,and strength. In a second embodiment, rotor winding 114 may be made ofaluminum or an aluminum alloy, which offers technical advantages interms of having a higher strength to weight ratio than copper, butperhaps at the expense of conductivity. In a third embodiment, rotorwinding 114 may be made of silver or a silver alloy, the former of whichoffers the technical advantage of being more conductive than eithercopper or aluminum, but perhaps at the expense of cost.

In other embodiments, rotor winding 114 may be made of an alloy orcomposite of any of those metals and flake graphene. Adding graphene mayoffer additional technical advantages in terms of added strength andelectrical conductivity, but perhaps at the expense of added cost andmanufacturing difficulties.

Graphene-metal composite may be produced in several ways. In one method,graphene flakes are mixed with copper powder and incorporated with oneanother through ball milling. The resulting mixture may be centered andcompressed into the composite. In another method, the composite may beproduced by 3D printing, or additive manufacturing, to apply layers ofgraphene flakes and copper powder, perhaps with a binding agent. Asintering process may follow to solidify the composite and drive out thebinding agent, by means of ball milling graphene flakes and copperpowder.

As mentioned above, each of the cavities 544A . . . N in rotor winding114 has a similar shape and orientation to accept cores with the samesize. Each core may be substantially equidistant from an axis of therotor. The cavities may be angled at a skew angle. The skew angle may beselected to minimize torque ripple and to ensure that the forces areapplied to the rotor winding and rotor core relatively evenly.

As mentioned above, the rotor winding and rotor cores may be made ofdifferent materials having different coefficients of expansion. Thecoefficient of thermal expansion is an intrinsic material property thatis indicative of the extent to which a material's volume, and therebylinear dimensions, changes with temperature. Different substances expandby different amounts.

Turning to FIG. 2, a band, such as band 108 in FIG. 1, is alsofabricated at step 202. Band 108 engages an outer edge of the rotorwinding and applies compression to the rotor winding. Band 108 is acircular band centered on an axis with a void sufficient to containrotor winding 114. The band may be made of a maraging steel. In otherexamples, the band may be made of a titanium or aluminum alloy, orcarbon fiber composite. Band 108 may also be formed by forging, hotrolling, or may be cold rolled and then welded into shape.

Band 108 is in tension owing to an interference fit. Band 108 applies aradial compressive stress such that, within the outermost regions of therotor, the compressive stress fully or partially cancels out the tensileforces originating from centrifugal acceleration forces acting on theupper portions of the rotor. A function of the rotor band is to preventexcessive radial deformation of the rotor. These compressive forces mayact to increase the allowable cyclic loading level (some combination ofeither the maximum angular velocity or number of cycles to a particularvelocity) of some portion of rotor winding 114. It is important to note,however, that the design of winding 114 is such that it prevents saidpreload compressive force from adversely affecting the enclosed cores.

Returning to FIG. 2, a thermal differential is applied between theassembly and a band at step 204. The thermal differential may involvecooling the assembly to contract it or heating the band to expand it, orboth.

While the thermal differential is applied the winding assembly isinserted inside a band at step 206. Step 206 is illustrated in FIG. 6.FIG. 6 shows a diagram 600 illustrating band 108 installed into a rotorfor an axial flux motor. Diagram 600 illustrates an assembly 604including a rotor winding with supports and cores inserted into itsrespective cavities. Assembly 604 is inserted into the void within band108. Assembly 604 also has lobed splines 602 to enable the rotor tomount on a shaft as will be discussed below with respect to FIGS. 17 and18A-B.

When the thermal differential dissipates and the band and rotor windingare at similar temperatures, band 108 applies a compressive force to therotor winding. In an example, band 108 may apply between 80 and 300megapascals of pressure to the rotor winding. Band 108 applies a radialcompressive stress within the outermost regions of the rotor. Again,these compressive forces may act to increase the allowable cyclicloading level of some portion of rotor winding 114. And, said rotor bandprevents excessive radial deformation of the rotor at high angularvelocities. In this way, method 200 creates a rotor for an axial fluxinduction motor that is more durable and uses materials that are highlyconductive and magnetically soft. Thus, band 108 increases the availableoperating speed of the machine.

In some embodiments, the band has portions removed to balance the rotorwhen the rotor is spinning. Removing portions in this way from afinished rotor may help align and balance and correct for anyimperfections during the manufacturing process. The result is a rotorthat has an equal distribution of mass around its axis. In this way, theband may be used to help avoid any wobbling when the finish rotor isspinning.

In some embodiments, the band has appendages to dissipate heat from therotor. Heat is conducted from the rotor winding to the band, includingto the appendages. Convection from the spinning rotor causes heat todissipate from the appendages to the surrounding air or other gas. Inthis way, the appendages may circulate air to cool the rotor when therotor is spinning.

Returning to FIG. 2, supports are applied to the cores at step 208 andthe cores are inserted into the rotor winding according to step 210. Thesupports may be added before or after the cores are inserted asdescribed above.

At step 208, supports are applied to suspend cores in respectivecavities of the rotor winding. As discussed above, the cores, such ascore 106 in FIG. 1, may be brittle and of low tensile strength.Secondly, for electromagnetic design reasons, it may not be possible tomake the rotor core in a shape that prevents bending or tension underacceleration and/or vibration loading. Finally, the thermal expansioncoefficient of the rotor inserts is often rather different from that ofthe conductive rotor material. For these three reasons, it is thereforedifficult or impossible to realize a direct interference fit betweenrotor and rotor cores that works across all temperature ranges that themotor or generator may experience (from, e.g., −40° C. to +200° C.

Moreover, as mentioned above, the coefficients of expansion for SMCcomposite and the material of the rotor winding are different. If theSMC composite were merely inserted, after applying a large enoughtemperature differential, to the rotor winding, the unequal compressiveforces applied by the rotor winding might destroy the rotor core.Alternatively, again because of the different coefficients of expansion,the SMC composite may become too loose when the rotor winding is heated.Embodiments disclosed herein avoid this problem by using alternatefixing methods, for instance, suspending the cores using supports asillustrated, for example, in diagram 400 in FIG. 4.

Diagram 400 includes rotor winding 114 that, just as in FIG. 1, containsin its cavities a number of cores, such as core 106. Core 106 isattached to rotor winding 114 by respective supports 404 and 406.Support 404 is an outer support that attaches to an outer portion ofcore 106 toward a perimeter of the rotor, that is, its outer shortingring. Support 406 is an inner support that attaches to an inner portionof core 106 toward the rotor's inner shorting ring. Support 404 may havea width between core 106 and rotor winding 114 of between 0 and 1.5 mm.Support 406 may have a width between core 106 and rotor winding 114 ofbetween 0.1 and 5 mm.

Supports 404 and 406 are made of a material that is rigid enough to holdthe cores and at the same time accommodate the different coefficients ofexpansion for the cores and winding. Thus, the material must be flexibleenough to adapt to different manufacturing tolerances of the cores andwinding. Example materials include an elastomer, such as latex rubber.More specifically, Fluroelastomer or FKM may be used. Fluroelastomer orFKM is a class of synthetic rubber designed for very high temperatureoperation. FKM provides resistance to chemicals, heat, and oil whileproviding useful service life above 200° C. FKM is not a single entitybut a family of fluoropolymer rubbers. Fluoroelastomers or FKM(sometimes also referred to as FKM Viton) can be classified by theirfluorine content, 66%, 68%, and 70% respectively. This means that FKMrubber having higher fluorine content has increasing fluid resistancederived from increasing fluorine levels. In a preferred embodiment, amaterial having a shore hardness of 90 A or above may be desired.

The elastomer may be vulcanized, such as by hardening by treating itwith sulfur at a high temperature. The vulcanization may utilize avariety of agents such as thiocarbanilide, thiourea, cumene peroxide,magnesium oxide, etc.), and in some instances, none at all (one-partcure).

In other examples, different substances may be utilized. For example,supports 404 and 406 may be made of a thermoplastic polymer likepolyether ether ketone (PEEK) or polyethylenimine (PEI). In otherexamples, supports 404 and 406 may be made of a high-temperature epoxy,acrylate or cyanoacrylate.

In one embodiment, the material for supports 404 and 406 may be appliedto core 106 before insertion into winding 114. In another embodiment, itmay be applied to winding 114 first before core 106 is inserted. In athird embodiment, it may be applied after core 106 is inserted. In thatcase, when vulcanized elastomer is used for supports 404 and 406, theentire core-winding assembly may be treated to complete thevulcanization process. This may be a mold-in-place operation. With amold-in-place operation, after the cores are inserted into the variouscavities on the rotor winding, elastomer, or other substance, may beinjected under pressure, in liquid form, to fill gaps present in theinner and outer portions of the cavities. Then, the entire assembly maybe heated to the vulcanization temperature to solidify the material.

Between the inner and outer portions of core 106 are side portions 408and 410. Alongside portions 408 and 410, an air gap may be allowedbetween core 106 and winding 114. In this way, the supports suspend therespective core in its respective cavity. Preferably, the air gap shouldbe just large enough to allow for manufacturing tolerances, and nolarger. In this way, space reserved for electromagnetic materials ismaximized.

Supports 404 and 406 may have various different shapes and may beapplied to suspend core 106 in various different ways, as will bedescribed below with respect to FIGS. 5A-D. Supports 404 and 406 may beshims, staves, stakes, or springs.

FIG. 5A illustrates rotor winding 114 with a plurality of cores 106A . .. N suspended using shims. A shim is a thin strip of material used toalign and secure parts, making them fit together in a desired fashion.As described above with respect to FIG. 4, cores 106A . . . N areattached to rotor winding 114 with inner and outer shims 504 and 506.Shims 504 and 506 may be rounded on one side to match a shape of acavity of rotor winding 114 and may be rounded on another side to matcha shape of the corresponding core 106. Shims 504 and 506 may be attachedto core 106 using an adhesive. Then, a shim-core assembly 514 may beinserted into a respective cavity in rotor winding 114. In this way,while an interference fit may be impossible between cores 106A . . . Nand windings 114 directly, an interference fit may be possible betweenwinding 114 and shims 504 and 506, and between shims 504 and 506 andcores 106A . . . N.

FIG. 5B illustrates a diagram 550 with another embodiment where thesupports are staves. A stave is a piece of material that interlocks withparts to secure them together. As in diagram 500 in FIG. 5A, diagram 550includes a plurality of cores, such as core 514, inserted into rotorwinding 502. Also as above, the plurality of cores are attached towinding 502 using inner and outer staves 516 and 518. The staves here,however, are shaped to mate with winding 502 and core 514. In this way,staves 516 and 518 extend into orifices in the respective cavities ofwinding 502 and the respective cores 514 to maintain the cores 514attached to and stationary within rotor winding 502.

In embodiments, the orifices may be grooves, spheres, or both cut ormolded into rotor winding 502, core 514, or both. Staves 516 and 518 mayhave shapes matching the respective orifice. The orifices andcorresponding staves may be placed both on the inner portion of core 514and outer portion, as illustrated at 520. In this way, the correspondingstaves and orifices mate with one another, making the core more secure.

Respective orifices on the rotor winding 502 and core 514 may join toone another to form a tube extending into the interior of a surface of arotor. A stave may be fabricated by injecting a polymer material, in itsliquid form, into the tube between core 514 and rotor winding 502. Onceinjected, the polymer material would solidify and harden to create asecure fit.

FIG. 5C illustrates a diagram 570 with a third embodiment using stakingto secure the core in the winding. Diagram 570 includes a rotor winding580 and a core 574. In diagram 570, core 574 is held in place by rings572A-D. Here, staking is used to keep in place the core. Staking is justan operation done on the copper rotor at the perimeter of the cavityedge, on both the bottom and top. Staking involves pressing on thisledge, causing the ledge or ring to deform to create a permanentencasing ring feature around the core. These rings fit into thecorresponding chamfer relief with which the inserts are made.

In this embodiment, rings are formed as 572A-D outcroppings from rotorwinding 580 by pressing along the edges of the rotor cavity. Rings572A-D may be made of the same materials as the rotor winding,preferably a chrome-copper alloy. For example, rings 572A-D may also bepressed and then post-machined to tolerances.

Rings 572A-D are configured to extend and fit into respective notches576A-D. Notches 576A-D are located at the corners of core 574. Notches576A-D may be beveled edges of core 574. When core 574 is pressed toshape, notches 576A-D may be formed from the die.

Rings 572A-D are strong enough to retain core 574. In this way, rings572A-D may in some instances maintain an interference fit with core 574.As core 574 and rotor winding 580 heat, they expand at different ratesaccording to their different coefficients of expansion. Rings 572A-D canslide against notches 576A-D to accommodate the different deformationsof core 574 and rotor winding 580. In this way, rings 572A-D continue topress against notches 576A-D in core 574, holding core 574 in placewithin rotor winding 580 as the rotor is heated.

FIG. 5D illustrates a diagram 590 with a fourth embodiment where thesupports are springs. Diagram 590 includes a rotor winding 591 holding aplurality of cores, such as a core 592. Each of the plurality of coresis held in place within a cavity in rotor winding 591 using a spring,such as spring 593.

Spring 593 may be a resilient device that can be pressed or pulled butreturns to its former shape when released. Spring 593 may be compressedwhen core 592 is inserted. Once inserted, spring 593 will exert (nearly)constant pressure on core 592 against a cavity within rotor winding 591.In a different example, spring 593 may be a helical compression spring,disc or Belleville spring. Spring 593 may be made of a resistive,non-ferromagnetic metal.

As rotor winding 591 and core 592 are heated, spring 593 accommodatesthe difference in sizing between the two pieces, keeping core 592 fitsnugly within rotor winding 591. In this way, spring 593 maintains core592 within rotor winding 591 despite differences in coefficients ofexpansion between the two pieces.

While method 200 is illustrated with an example rotor illustrated inFIG. 1, a skilled artisan would recognize the same manufacturingtechnique may be used to manufacture other rotors having similarproperties.

While various supports are described to attach cores to the rotorwinding, a skilled artisan would recognize that, in other embodiments,the cores may be attached to the rotor winding using soldering orbrazing techniques.

As mentioned above, an adhesive may be used to attach the support to thecore and winding. In another embodiment, the cores may be attacheddirectly to the winding with an adhesive. In that embodiment, a gap ofroughly 0.5 mm between the core and the rotor winding may be filled inwith adhesive. In examples, the gap may be between 0.3 mm and 0.7 mm inthickness. In either case where a support is used or an adhesive is useddirectly, the adhesive may be an epoxy adhesive such as EP-830. Theadhesive may have a glass transition temperature of at least 175° C. anda temperature rating of at least 180° C.

Stator for an Axial Flux Induction Machine

FIG. 7 illustrates a stator 700, similar to stators 132A-B in FIG. 1,including a stator core and stator windings according some embodiments.Stator 700 includes stator core 112 and stator windings 116.

As described above, stator core 112 may be made of SMC or othermagnetically permeable material. It may have a base and a number ofteeth, such as tooth 730. The teeth are separated by slots, such as slot732. The teeth protrude upward from the base and the base shortsmagnetic circuits between the respective teeth. In a preferredembodiment, stator core 112 may have 36 teeth.

Surrounding the teeth are stator windings 116. Stator windings 116include a plurality of coils such as coils 712, 714, 716, 718, 720, and722. In a preferred embodiment, stator windings 116 may have a number ofcoils that corresponds to a number of teeth in stator core 112. Forexample, in a preferred embodiment, stator core 112 may have 36 teeth.Thus, stator windings 116 may have 36 coils.

The coils in stator windings 116 may have three phases, eachrepresenting a different, closed electrical circuit. The coils in eachphase make up a coil group. In the preferred embodiment shown in FIG. 7,two coils having a common phase are placed immediately next to eachother (making a coil pair) following the two alternate phases. Forexample, coil 714 is adjacent to coil 712. Both coil 714 and 712 arepart of a first phase, labeled phase 702A. That pair of coils in phase702A is adjacent to a pair of coils in a different phase, phase 702B.The adjacent pair of coils in that phase is coils 716 and 718. Followingthose coils is a third pair of coils, coils 720 and 722, in a thirdphase, phase 702C. This pattern, AA, BB, CC, repeatedly continues aroundthe stator for all 36 coils.

FIG. 8A illustrates a coil 800 for stator windings according to someembodiments. Coil 800 may be insulated rectangular wire. In oneembodiment, wire may be made of ETP or OFHC copper. The width of thewire may correspond to a width of the slots. For example, each slot maybe wide enough to fit a single wire across plus a small allowance forinsulating paper and manufacturing tolerance.

Coil 800 has a plurality of turns, such as turn 804. Each turnrepresents a loop of the winding back to its original lateral point. Inthe preferred embodiment in FIG. 8A, a coil may have five turns, all ofcommon shape. The number of turns corresponds to a thickness of therectangular wire in the axial direction and a height of the stator teethto maximize the volume of space in the slot occupied by copper.

In this embodiment, turn 804 is bent to enclose four stator teeth. Basedon the number of teeth that the coil encloses, turn 804 may havedifferent geometries and shapes. In the embodiment shown in FIG. 8A,coil 800 is bent at angles 806 and 808 such that a segment 807 of wirebetween angles 806 and 808 extends a radial direction (that is, adirection extending from an axis along a radius of a machine). Thissegment of wire is positioned to pass between two stator teeth, thoughthe slot. At bend 810, coil 800 is bent 180° between a direction facingthe axis of the machine back toward a direction facing the perimeter ofthe machine. Extending back around turn 804, coil 800 is bent at angles812 and 814 such that a segment 813 between angles 812 and 814 alsoextends a radial direction. Finally, coil 802 finishes turn 804 at itsinitial position in the radial dimension.

To form turn 804, coil 800 extends outward in alternating directions ineach half-turn: first in a direction towards angles 806 and 808 andsegment 807, then in a direction towards angles 812 and 814 and segment813. These alternating directions provide the ability for the coils tointerlace and lap on top of one another. In other words, a segment 807of an adjacent coil (not shown) can rest on top of segment 813. This canbe extended to several different coils. For example, three or four coilsmay be interleaved into coil 800 when assembled into the statorwindings. By having the segments that extend out radially from the axisof rotation, the windings form a unique “truncated pie wedge” shape.

These coils then have their free ends bent into tabs. These tabs allowthe coil ends to be welded or brazed to interconnect bars (or wires).

FIG. 8B illustrates a coil 850. Coil 850 is similar to coil 800 in FIG.8A. However, coil 850 has ends 852 and 854 that do not return to thesame lateral position, but instead extend in a radial direction awayfrom the rotor. This has the effect of changing how interconnects may bewelded to connect different phases of the stator winding.

FIG. 9 illustrates a method 900 for manufacturing a coil, such as coil800 in FIG. 8A, for a stator winding according to some embodiments. Thisis one embodiment. In other embodiments, the coils and theirinterconnections can be made with additive manufacturing techniques.Also, the coils and/or their interconnects may be cast. After casting or3D printing, the metal may be dipped into a substance that serves asinsulation. For illustrative purposes, method 900 is described withrespect to examples in FIGS. 10A-K. FIGS. 10A-K are diagramsillustrating an example operation for the method for manufacturing thecoil.

Method 900 begins by de-coiling wires from a spool at step 902. This isillustrated in diagram 1000 in FIG. 10A. Diagram 1000 shows a spool 1002with two pulleys 1006 and 1008. Wire 1004 is spun out from spool 1002around pulleys 1006 and 1008. Pulleys 1006 and 1008 are friction devicesthat serve to keep tension on wire 1004 to prevent overrunning as wire1004 un-rolls from spool 1002.

After de-coiling, the wire is straightened at step 904 as illustrated indiagram 1010 in FIG. 10B. To straighten the wire, it is passed through atwo plane straightener. The first plane is illustrated by straightener1012 and the second plane is illustrated by straightener 1014. Eachstraightener includes a plurality of rollers oriented on either side ofwire 1004 to straighten the wire along its respective x or y dimension.In this way, straighteners 1012 and 1014 eliminate the set of the coiland present consistent material to follow on coil forming devices.

After the wire is straightened, the wires are cut to length at step 906.To measure and feed the appropriate length of wire to subsequentprocesses, a feed unit is illustrated in diagram 1020 in FIG. 10C.Diagram 1020 includes a feed unit 1022. The unit 1022 is servocontrolled to feed each leg of the coil to the desired length. Thelength of the coil can be, e.g. programmed using the operator HMItouchscreen, or it can be saved in the controller memory or programmedvia a networked control supervisory program (such as a CODESYS, TwinCAT,RSLogix, or DeltaV supervisory control program). The length of the coilmay be determined based on the size of the turn, the size of the statorteeth, the number of teeth the coil encapsulates, and the number ofturns in the coil. Turning to diagram 1030 in FIG. 10D, the wire may becut using a pair of wire cutters 1038.

Returning to method 900, the wire is bent into a stack-up in an axialdirection (along an axis) of the machine in step 908. To form thestack-up, the wire is fed through a bend unit shown in diagram 1030 inFIG. 10D. The bend unit is servo controlled for precise degree rotation.The bend unit includes a following die 1032. Following die 1032 has anangle 1036. Once the wire has been advanced to a sufficient degree, thatis, the amount of wire needed to complete half a turn, a cam 1034presses the wire against die 1032 forming a band having an angle 1036.The result is shown in diagram 1040 in FIG. 10E, which shows a wirehaving a zigzag pattern with a number of angles, such as angle 1042.

The bent wire with a zigzag pattern shown in diagram 1040 is thenpressed into a stack-up as shown in diagram 1050 in FIG. 10F. The resultis shown in diagram 1060 in FIG. 10G. The stack up is bent back andforth in an axial direction. The number of bends corresponds to thenumber of electrical turns in the coil. For example, the stack up mayinclude two layers for every turn. Thus, in the case where the coil hasfive turns, the stack up shown in diagram 1060 has 10 layers, and isbent nine times.

Returning to FIG. 9, the bent, stacked wire is transversely pressed witha die having a pattern of alternating forming teeth at step 910. Such apress is illustrated in diagram 1070 in FIG. 10H. Diagram 1070illustrates a forming unit. The forming mechanism may have clamp dies1072A and B that hold the stack-up in place. Clamps 1072A and B may becontrolled by servo systems 1074A and B. Clamps 1072A and B may hold theends of the stack, when oriented in a radial direction.

With the wire stack clamped on both longitudinal locations (at the endturn folds), the pressing dies 1076A and B may press the wire stack inthe transverse direction, corresponding to the direction of rotation ofthe machine. As the press pushes in, the clamp moves in as well torelieve the stress in the radial direction caused by pressing dies 1076Aand B. Pressing dies 1076A and B are powered by servo mechanisms 1078B.Pressing dies 1076A and B have alternating sets of forming teeth. Theshape of these forming teeth are designed such as to form the wire tofit over at least one tooth of the stator. How the alternating sets offorming teeth create the form of the part during pressing is illustratedin greater detail in diagrams 1085, 1090, and 1095 in FIGS. 10I-K.

FIG. 11 illustrates a method 1100 describing how coils are interleaved,interconnected, and inserted on an open stator.

Method 1100 begins at a step 1102 by fabricating coils and stator cores.The coils may be fabricated as described above with respect to FIG. 9.As mentioned above, the stator core could be made of a laminated siliconsteel or a soft magnetic composite (SMC) material. As described above,if the stator core is made of SMC, it may be manufactured by repeatedlycompressing the SMC powder it into a die having the correct shape untilit reaches needed density. When such a density is achieved, the greenpart is then exposed to the two-part heat and steam treatment outlinedabove.

After the coils are formed, a set of them are inserted into aspecialized jig at step 1104. At step 1104, the coils are inserted, oneat a time, into a jig where they are held in correct position to bewelded to interconnects and then later fit into the stator as anassembly. This is illustrated in FIGS. 12A-B.

FIGS. 12A-B illustrate using a specialized jig to assemble the coilsinto a winding. FIG. 12A is a diagram 1200 showing a number of columns1202A-F, which are part of the specialized jig. The jig may have anumber of columns corresponding to the number of coils. For example, fora winding that has 36 coils, the jig may have 36 columns.

Each of columns 1202A-F may have a clamp that holds a coil, such as coil1204, in place. Each of columns 1202A-F can extend and retract to allowcoils to be slid or rotated into their laps or interlaced position. Indiagram 1200, column 1202A is extended and clamps a coil 1204. Coil 1204may be the first coil of the winding assembled. All the other columns(e.g., 1202B-F) are retracted.

Turning to diagram 1210 in FIG. 12B, another coil 1216 is slid androtated within the plane of the rotation of the machine to interleavewith alternating turns from coil 1204. Coil 1204 laps over coil 1216.Coil 1216 is moved to the correct axial location and then rotated intothe existing coil group. Once coil 1216 is positioned correctly, it isheld stationary by column 1202B, which clamps it in place. After coil1216 is in place, column 1202B is extended to clamp coil 1216 in place.The process is repeated for each coil until all coils in the winding arein place. The columns may extend repeatedly as each coil is positionedand may resemble teeth of the stator. In this way, the columns may holdthe coils in place.

Returning to FIG. 11, once all of the coils are inserted, the coils arethen electrically connected to the interconnect bars/wires by (eitherlaser, electrical resistance, or ultrasonic) welding at the coil tabs atstep 1106. The welding is discussed below in greater detail with respectto FIGS. 12C-D.

After interconnect welding, the coil assembly is then finally insertedinto the stator at step 1108. This may be done by first installing theslot insulation (Kapton film and/or Nomex 410/411 paper and/or bondedMica tape) over the windings and tack gluing them into place while thestator winding rests on a locating table. The slot insulation protectswire insulation from damage when windings are overlaid and improvesinsulation between stator phase coils and the conductive stator, whichcan be at ground potential. The slot insulation, once fully potted, alsodampens vibration and restrains the movement of the coil end turns.

The stator core is then picked up by a retractable mandrel or arborthat, respectively, would contact the inner or outer base portion of thestator, and would then be inserted upside down over the coil assemblyuntil it fully seats against the jig table face. The stator manipulatoris then released and removed from the stator. Once the stator is seated,the stator is compressed/sandwiched against the jig table face by aplate on the back side, and this compression jig is then used totransport the coil assembly to the potting step, where the stator andcoil assembly are potted with, e.g., a high temperature epoxy orpolyimide potting. Prior to this step, the windings may also have aNomex or Kevlar tow lacing applied to further restrain movement andstrengthen the assembly.

Once the stator core is vacuum impregnated with varnish and/or is pottedwith a filled or unfilled resin (e.g. high temperature epoxy), and aftersaid resin or potting has cured, it is then released from the sandwichjig, has dimensions checked (via, e.g., CMM, Go/No-Go gauges, opticalinspection, height comparator, etc.) and may have finish machiningoperations applied as needed to, e.g., true the axial faces of thestator, size to final dimension, etc. Once these operations arecomplete, the stator is ready to again have the manipulator inserted toready it for placement into an endbell. Shims for the stator are firstinserted into the endbell, and the stator is then placed atop of theseshims and is then affixed to the endbell by the appropriate fasteners.

FIG. 12C illustrates a diagram 1200 with two coils 800A and 800B lappedon one another with interconnects welded onto joints. For example, coil800A has interconnect 1222C welded at an upper joint 1226B. Upper joint1226B represents the end of the coil closest to the rotor. Coil 800B hasinterconnect 1222B welded at a lower joint 1224A. Lower joint 1224Arepresents the end of the coil farthest from the rotor. The joints maybe welded by first removing insulation in the desired region, perhaps bylaser etching or mechanical abrasion (sanding or scraping), and thenwelding the interconnects to the coil end. The interconnects may berectangular wire having the same dimensions as the wire used for thecoil, or larger dimensions to decrease losses. The interconnects may beshaped to be joined with coils from the same phase and to extend thephase circuit around a perimeter of a stator winding. Laser welding,resistance welding/brazing, ultrasonic welding, or manual TIG or gastorch welding or brazing may be used as the joining method.

FIG. 12D illustrates a diagram 1250 that shows a single phase of astator winding interconnected together. Adjacent coils 1252 and 1254 areconnected with interconnects 1253. The pair of adjacent coils 1252 and1254 are connected with another adjacent pair of coils 1256 and 1258 viaa longer interconnect 1255. This pattern continues to join all of theapplicable coils (in this instance, 12 coils) into the phase group(there are 3 phase groups in the overall stator coil assembly). Joints1260 and 1262 represent points that are loaded with a periodicallyvarying electrical signal as illustrated in FIG. 13.

FIG. 13 illustrates a diagram 1300 showing example signals applied tothree phases of the stator coils. The signals shown here areillustrative of a three phase machine. In other embodiments, a differentnumber of phases may be present. In particular, diagram 1300 shows asignal 1302, a signal 1304, and a signal 1306. Each of signals 1302,1304, and 1306 may be periodically varying electrical signals, such assine waves. The sinusoidal signals 1302, 1304, and 1306 are offset by120° of phase from one another. In embodiments having a different numberof phases the offsets may be different. By varying the amplitude,frequency, and absolute phase of these signals (but not their relativephase angle relationship), operation of the electric motor may becontrolled.

The electromagnetic torque depends on stator currents and slip. Slip isthe ratio between the shaft angular velocity and the synchronousmagnetic field angular velocity. Slip is a result of mechanical loadapplied to the shaft. In general, the more voltage that is applied tothe stator winding, the more current is generated and the more torquecreated. More slip has the same effect. When the load applied to theshaft is increased the slip is increased too. More currents are inducedin the rotor winding and more currents are produced in the statorwinding as the impedance of the rotor-stator equivalent circuit reducesas the mechanical loading increases.

For example, increasing the voltage of input signals 1302, 1304, and1306 results in more current passing through each phase of the statorwinding. More stator current in turn produces more magnetic flux, andmore magnetic flux induces more rotor currents and hence more torque, aswill be described below in more detail with respect to FIGS. 22A-D.Thus, given a constant slip value, more voltage in signals 1302, 1304,and 1306 will produce more torque.

However, if the torque applied to the motor is changed while the voltageis held constant, the slip value will change. This is because, given aconstant voltage, a reduced externally applied torque will result in anincreased rotational velocity (e.g., rotations per minute) of the rotor.Thus, if the frequency of the signals passing through that statorwinding is held constant, the slip will decrease because the shaft isrotating more quickly. Decreased slip will result in decreased rotorcurrents and torque generated.

If more external mechanical load is applied (and the voltage andfrequency of the signal applied to the stator winding is held constant),the shaft's rotational velocity will decrease and the slip willincrease, generating more torque. To avoid a decrease in speed, rotoroverheating, or reaching the rotor's tripping point, the voltage andfrequency of the signal in the stator winding can be increased. In thisway, a circuit that generates the signals for the stator winding cancontrol torque and rotational speed by calculating an appropriatevoltage and frequency of the signal in the stator winding based on arotational speed of the shaft detected based on a position sensor. Thecircuit may be configured to calculate the appropriate voltage,phasing/commutation, and frequency to minimize the heat generated in themotor.

There may be situations when the rotor is locked; that is, the rotor isnot spinning at all. This may occur, for example, when a vehicle is justbeginning to move. When a position sensor detects a locked rotorcondition, the voltage and frequency may be slowly ramped up from zeroto produce the needed torque for adequate acceleration while minimizingpower consumption and thus heat production.

When a position sensor detects movement from a locked rotor, ideally thevoltage and frequency would ramp up as a linear function of shaft speed,keeping electromagnetic torque at the needed level to accelerate to theneeded speed.

In an embodiment, the circuit that generates the signals applied to thestator winding may be an inverter that converts DC voltage from, forexample, a battery. In another embodiment, the signals may be generateddirectly from the 50 or 60 Hertz signals found on the power grid.Embodiments using grid-tied power may be preferred for applicationsinvolving, for example, household appliances.

Mounting a Stator to an Endbell

FIG. 14 shows a stator core 1400. Stator core 1400 may be a unitarystructure or may be formed of layers. For example, stator core 1400 maybe made of SOMALOY to reduce eddy currents in stator core 1400. Statorcore 1400 may also be formed from laminated layers of steel, iron, orother magnetic materials. The use of laminated layers of magneticmaterials may also reduce losses due to eddy currents during operationof the motor.

In some embodiments, teeth 1402 extend from a base 1404 of stator core1400. Teeth 1402 may have straight, flat sides and may narrow inwardlyas one moves in the radial direction toward a center point of statorcore 1400. For example, tooth 1402 may be narrower nearer axis ofrotation 1412 and may be thicker further from axis of rotation 1412. Insome embodiments, teeth 1402 may widen at the top (which is axially awayfrom the base), configured to both retain windings 116A-B and to reducemagnetic flux density spikes. The space between adjacent teeth 1402defines slots 1410. Slots 1410 are configured to receive windings116A-B. A retaining lip 1408 may extend from base portion 1404.Retaining lip 1408 is configured to mate with additional structuralelements to retain stator core 1400, for example, in an endbell 1500mounting as described further below. Stator 1400 may also be retainedusing notches 1406 configured to receive fasteners. In some embodiments,notches 1406 may be holes or voids cut into base 1404 or retaining lip1408.

FIG. 15 shows stator core 1400 mounted in an endbell 1500. Endbell 1500has a side wall 1502. Two endbells 1500 may come together to form anendbell housing, as illustrated with endbells 102A and 102B in FIG. 1.

Stator core 1400 is held stationary within endbell 1500 using a bracket1504. Bracket 1504 attaches to the endbell and engages with the lip ofstator core 1400 to hold stationary stator core 1400 in endbell 1500. Inan embodiment, bracket 1504 may hold stator core 1400 flush to endbell1500 as shown at 1506. In another embodiment, shims may be used betweenstator core 1400 and endbell 1500 to ensure that stator core 1400 ispositioned correctly within endbell 1500. By using a bracket 1504 toattach stator core 1400 to endbell 1500, embodiments avoid the need tomachine stator core 1400, which could adversely affect its magneticproperties.

FIG. 16A shows endbell 1600. Endbell 1600 has an axle shaft hole 1608through which a shaft 140 may pass. Endbell 1600 also includes aretaining ridge 1604 and fastener receiver 1606. Retaining ridge 1604 isa guide that allows for proper positioning of bracket 1504 withinendbell 1600. To provide proper positioning, retaining ridge 1604 mateswith bracket 1504 and interlocks with it at the proper position in theendbell.

In some embodiments, fastener receiver 1606 may be a through-hole inendbell 1600 configured to receive fasteners 1630. Fasteners 1630 mayextend through fastener receivers 1606 to an exterior of endbell 1600. Aportion of fasteners 1630 on an exterior of endbell 1500 may then besecured using any one of a variety of techniques. For example, fasteners1630 may be secured using threaded members, such as nuts or screws. Asan advantage of using a threaded member, shimming may be used to adjustthe position of the stator in endbell 1600. In an alternativeembodiment, fasteners 1630 may also be secured using welding.

Turning to FIG. 16B, fasteners 1630 may be secured to bracket 1504 tosecure stator core 1400 to endbell 1600. Bracket 1504 may have a lipportion 1632 configured to engage retaining lip 1408 of stator 1400. Lipportion 1632 and retaining lip 1408 may be formed such that the twotightly mate. When bracket 1504 is laid over stator 1400 in endbell1500, bracket 1504's lip portion 1632 may engage retaining lip 1408 andfasteners 1630, which may extend from bracket 1504 and pass throughfastener holes 1606.

Fasteners 1630 may be rigidly coupled to bracket 1504. In someembodiments, fasteners may mate with a slot or other member of bracket1504 such that fastener 1630 does not disengage from bracket 1504. Insome embodiments, bracket 1504 couples stator 1400 to endbell 1500 usingonly a friction connection. The tightening of fasteners 1630 on theoutside of endbell 1500 tightens the connection between endbell 1500 andstator 1400 by using a mechanism to secure fasteners 1630 on theexterior of endbell 1500. This connection limits the number of itemsthat could break or otherwise become dislodged during operation, whichcould interfere with the rotation of the motor.

In some embodiments, bracket 1504 may have a coating 1628 covering anon-ferromagnetic metal. The non-ferromagnetic metal and stator core1400 may have different thermal coefficients of expansion. The coatingbetween the non-ferromagnetic metal and the lip may be configured suchthat the stator core remains attached to the endbell at a givenoperating temperature range of the machine given the differentcoefficients of expansion of bracket 1504 and stator core 1400. In anembodiment, coating 1628 may be made of vulcanized rubber or othersuitable thermoset elastomer.

As mentioned above, endbell 1600 includes a retaining ridge 1604 thatmates with bracket 1504. In particular, retaining ridge 1604 mates withan inner portion 1624 of bracket 1504 and is held in place with outerportions of bracket 1504, such as outer portion 1626. Bracket 1504 alsohas protrusions 1632 that mate with stator core 1400, aligning it to theproper orientation. In this way, bracket 1504 serves to ensure thatstator core 1400 lines up appropriately. Stator core 1400 may line upappropriately when respective phases of a stator winding engage with thecounterpart phases on the opposite stator in the rotating electricalmachine.

FIG. 16C shows an assembly 1640 with an endbell 1600 holding stator core1400 in place with a bracket 1620. FIG. 16D is an exterior 1660 ofassembly 1640 with fasteners from the bracket attached with nuts, suchas nut 1662.

Mounting a Rotor to a Shaft

FIG. 17 shows a diagram 1700 illustrating how a rotor is mounted to ashaft in a rotating electrical machine. Diagram 1700 includes shaft 140held in place with two bearing assemblies 1704A and B. Shaft 140 isaffixed to a rotor 1702 that applies torque to spin shaft 140.

Shaft 140 may be a unitary structure and made of high strength,non-ferromagnetic materials. For example, shaft 140 may be made of steelor steel alloy. In some embodiments, shaft 140 has features toaccommodate the mounting of additional components on shaft 140.

For example, as shown in FIGS. 17 and 18A, shaft 140 is shaped such thatit has two conical frusta extending from the midsection that truncate atshoulders 1706 and 1708 on shaft 140. A first side of the shoulder 1706abuts a bearing assembly 1704A. Bearing assembly 1704A is configured topermit the rotation of shaft 140 and is supported by the bearingcup/bore of endbell 1500. Another bearing assembly 1704B is locatedalong shaft 140. The second bearing assembly 1704B is also supported bythe bearing cup of endbell 1500 and also permits the rotation of shaft140.

FIG. 18A illustrates a cross-section 1800. As shown in cross-section1800, a locking member (lock ring) 1802 is placed up against rotor 1702to hold rotor 1702 in place and prevent it from moving along shaft 140.Locking member 1802 may be held in place by a dowel placed in hole 1804.Shaft 140 locks into rotor 1702. Lock ring 1802 and shaft 140 have dowelholes aligned in the theta and axial directions to permit preciselocation of the lock ring relative to the now entrapped rotor 1702 asillustrated in FIG. 18B.

FIG. 18B illustrates a rotor 1850 with a void 1852. Void 1852 iscentered along an axis of rotation 1860. Void 1852 is a curved lobedspline. The spline lobes may be sinusoidally shaped. Inner spline lobes1854A-E mesh with counterpart outer spline lobes on the shaft totransmit torque to the shaft.

The spline lobes may be shaped substantially according to the equation:

r(Θ)=r _(midline)+(r _(lobe_size)*sin(n*Θ));

where n is the desired number of lobes, Θ is an angle around axis ofrotation 1860, r_(midline) is a distance 1851 from the axis of rotation1860 to a median of the curved lobed spline, and r_(lobe_size) is anamplitude of the lobe, that is, the distance 1853 and 1855 betweenmaximum and minimum distances to axis of rotation 1860 or the differentbetween distances 1858 and 1862.

As mentioned above, sinusoidal splines 1854A-E are used as a means totransmit torque between the rotor and shaft. The use of a curved splinedramatically reduces stress concentration at the root of the rotor,where stresses are typically at a maximum due to centrifugal loads, andenables transmission of torque while reducing fatigue.

Various Alternatives

FIG. 19 shows a diagram 1900 illustrating another rotor assembly in analternate embodiment. Diagram 1900 illustrates a rotor winding 1902 witha plurality of rows of cavities extending radially outward on thewinding. Alternatively, the cavities could be at a skewed offset angle.In addition, the cavities may be organized in a series of concentriccircles at illustrated in FIG. 19. Each row, such as row 1904, includesa plurality of cavities. In one possible configuration, the cavities maybe hexagonal. As with the rotor in FIG. 6, this rotor could also includea band (not shown).

The rotor assembly in diagram 1900 may also perform well in avoidingeddy currents while producing torque in a compact design. At the sametime, this rotor assembly may have an advantage of working equally wellrotating in forward and reverse. This may be advantageous in motors forelectric vehicle applications.

Having described various rotors for axial flux induction rotatingelectrical machines and how they are assembled, this disclosure nowdiscusses how the rotors can operate in an example axial flux inductionmotor.

FIGS. 20-21 show axial flux motors according to some embodiments. Asdescribed above with reference to FIG. 1, an axial flux induction motormay comprise a rotor sandwiched between two stators. But the opposite,two rotors with a single sandwiched stator, is also possible asillustrated in FIG. 20.

FIG. 20 shows an axial flux induction motor 2000 with a stator 2002sandwiched between two rotors 2004 and 2006. In other embodiments notshown, the motor may include multiple stators and multiple rotorsalternating in position with one another. In still another embodiment,the motor may have a single stator and a single rotor.

FIG. 21 shows an axial flux motor 2100 according to some embodiments.Motor 2100 includes a rotor 2106 sandwiched between two stators 2102Aand 2102B. Stators 2102A and 2102B pass magnetic flux, illustrated forexample by flux lines 2104A . . . N, parallel to an axis of rotor 2106which runs along shaft 2110. Thus, motor 2100 is an axial flux motor.The magnetic flux changes over time as the magnetic flux rotates aroundthe stator. The changing magnetic flux induces currents in rotor 2106.The currents in rotor 2106 produce a magnetic flux, which interacts withthe magnetic flux produced by the stator, and thus torque is produced.This process is illustrated in greater detail with respect to FIGS.22A-D.

Operation of an Axial Flux Induction Motor

FIGS. 22A-D illustrate how an axial flux induction motor produces torqueaccording to some embodiments.

FIG. 22A shows a diagram 2800 illustrating a winding 2204A around atooth of stator 2202A and a winding 2204B around a tooth of stator2202B. Windings 2204A and 2204B may be simple copper wires. A currentperpendicular to and oriented INTO the plane of the drawing throughwinding 2204A creates (via the right-hand rule) a magnetic field 2206A,and a current perpendicular to and oriented INTO the plane of thedrawing through winding 2204B creates a magnetic field 2206B. Stators2202A and 2202B are made of a magnetically permeable material. Thus,magnetic fields 2206A and 2206B magnetize a magnetic circuit whichincludes magnetic flux lines 2104A and 2104B.

FIG. 22B shows a diagram 2220, similar to diagram 2200 in FIG. 22A. Inaddition to the components in diagram 2200, diagram 2220 illustrates arotor bar 2222 at time t₀.

FIG. 22C shows a diagram 2240, advancing to a later time t₁. The currentgoing through windings 2204A and 2204B is alternating current and issynchronized. Hence, at the later time t₁, current going throughwindings 2204A and 2204B has changed direction. Because current goingthrough windings 2204A and 2204B has changed direction, the magneticflux produced by the current and magnetic fields 2206A and 2206B hasalso changed direction since t₀. The changing magnetic fields 2206A and2206B change the magnetic flux through the stators, illustrated by fluxlines 2244A and 2244B. The changing flux induces a current in rotor bar2222. The current in rotor bar 2222 creates a magnetic field 2248 whichis in opposition to the magnetic flux 2244A from t₀ to t₁. This inducedcurrent in rotor bar 2222 creates a torque in accordance with theLorentz force law.

Note that torque is produced despite the fact that rotor bar 2222 maynot have moved. In this way, embodiments can create torque even in alocked rotor situation. In fact, given enough current, significantlocked rotor torque can be generated, at least over time periods shortenough for heat to dissipate. This provides an advantage over manytraditional radial induction motors having the same volume, which do notprovide as significant locked rotor torque. Radial induction machineshave a lower power density. Therefore, to produce equivalent lockedrotor torque, the machine would have to be much larger. Moreover, thislocked rotor torque is achieved without the need for permanent magnetsas is traditionally necessary to produce the starting torque illustratedin FIG. 22C with equivalent volume and mass in the machine. Avoidingpermanent magnets saves cost and avoids environmental damage needed toobtain the rare earth magnets. Also, permanent magnet motors fall off inefficiency at higher RPMs, because the magnetic field produced by thepermanent magnets is fixed. In contrast, the induction motorscontemplated in the current embodiment are more efficient at higherRPMs, since the rotor excitation can be changed to a needed value withstator voltages, currents and rotor slip.

Induction motors offer a number of other advantages as well. Forexample, in general they have a more flat torque curve, and importantly,have their torque maxima at close to synchronous speed, which is a verycomplimentary behavior to that of a PM motor. Another benefit ofinduction machines is the absence of parts which could be demagnetizedirreversibly by applied magnetic field with or without additionaltemperature. For PM machines, excessive magnetic flux produced by astator with or without additional temperature may cause reversible orirreversible demagnetization of PMs.

FIG. 22D is a diagram 2260 illustrating how additional torque is createdonce the rotor begins moving. In addition to the components in FIGS.22A-C, diagram 2260 also illustrates a core 2262. When the rotor has awinding which is only made of electrically conductive material which hasmagnetic permeability close to that of air, as shown on FIG. 22C, asignificant part of magnetic flux is dissipated on its way from stator2702B to stator 2702A and vice versa. This way the rotor winding iscrossed with less magnetic flux than what is produced by stators.

When ferromagnetic cores 2262 are placed into cavities of a rotorwinding, magnetic flux gets a path from stator 2702A to 2702B with lessmagnetic resistance as equivalent air gap length is significantlyshorter. This way less magnetic flux is dissipated and more magneticflux crosses the rotor winding in perpendicular direction to the rotorwinding turns which means the rotor winding gets more flux linkage(Psi), which is changed in time. Voltage induced in the rotor winding isequal to, or substantially corresponds to,

${relation}\mspace{14mu}\frac{dPsi}{dt}$

which means more flux linkage and more voltage is induced in the windingper single time step. Further the more voltage is induced in the rotorwinding the more current is produced in the rotor. And more current inthe rotor winding produces more electromagnetic torque in motor.

FIGS. 22A-D provide a simple example demonstrating how rotors accordingto the embodiment described herein can generate torque in an axialinduction motor. FIGS. 22A-D involve just two stator teeth, a singlerotor bar, and a single core.

FIGS. 23A-B show a magnetic field and an electric current in a rotor foran axial flux motor according to some embodiments. FIG. 23A shows adiagram 2300 illustrating a magnetic field, and FIG. 23B shows a diagram2350 illustrating an electric current in a rotor for an axial flux motoraccording to some embodiments. Diagram 2300 shows magnetic field lines2302 in a stator 2304. The magnetic flux induces currents 2305 throughrotor bars, such as rotor bar 2304, in th rotor. These induced currentsproduce torque in the same manner described for FIGS. 22A-D.

Cooling Axial Flux Induction Machine

FIGS. 24A-B show a cross-section of a rotating electrical machine 2400with an impeller attached to a shaft. The impeller operates to coolmachine 2400.

As described above with respect to FIG. 1, machine 2400 has stators 132Aand 132B, each including a respective stator core 112A and 112B and arespective stator winding 116A and 116B. Machine 2400 also includes arotor 134. Between each of the two stators 132A and 132B and rotor 134is an air gap, such as air gap 2410. Rotor 134 spins, applying torque toshaft 140.

As described above with respect to, for example, the operation of anaxial flux rotating electrical machine, the process of generating torqueinvolves generating current. As described above, alternating current isapplied to stator winding 116A and 116B, which generates a changingmagnetic field, which induces current in rotor 134. The currents, bothin stators 132A and 132B and in rotor 134, generate heat. If machine2400 gets hot enough, components could deform and/or break.

To deal with this heat, machine 2400 includes impellers 2402A and 2402B.Each of impellers 2402A and 2402B are mounted to shaft 140. In additionto being affixed to shaft 104, each of impellers 2402A and 2402B abutand are in contact with rotor 134. As illustrated in FIG. 24A, stators132A and 132B are each roughly circular and have an empty center wherethe coils are turned. Impellers 2402A and 2402B are located in the emptycenter portions of stators 132A and 132B.

Impellers 2402A and 2402B each have a plurality of fins, such as fin2404. The plurality of fins may be arranged substantially equidistantalong the circumference of shaft 140. Each fin is an appendage extendingalong the impeller in substantially the radial direction of machine2400. As shaft 140 spins, impellers 2402A and 2402B spin as well. Theplurality of fins move air through air gap 2410 and along shaft 140.Cool air moving along the surface of rotor 134 removes heat throughconduction and convection. Moreover, because the components are incontact with one another and may be made of materials that areconductive of heat, heat is transmitted from rotor 134 to impellers2402A and 2402B and their respective plurality of fins. Again, as airmoves along the channels between adjacent fins, heat is transferred fromthe fins to the air and forced out of machine 2400.

The plurality fins may be forward facing or backward facing depending ontheir orientation and the direction of rotation. FIG. 24A illustrates aforward-facing fin arrangement. Shaft 140 rotates in a counterclockwisedirection as illustrated at 2406. Moving inward toward the axis ofrotation of machine 2400, the position where each fin, such as fin 2404,is affixed to shaft 140 shifts in the direction of rotation, in thiscase, counterclockwise. Moving in that direction, air is pulled firstalong shaft 140 then along rotor 134 as shown by lines 2408A and 2408B.

Each fin may be angled such that it angles towards the radius of machine2400 at each end of the fin. The portion of each fin farthest from rotor134 (and closest to shaft 140) may taper off towards shaft 140. Theportion of each fin closest to rotor 134 (and farthest from shaft 140)may have an edge, such as edge 2412 substantially in the axial directionof machine 2400. The length of edge 2412 may be set according to thewidth of air gap 2410. In various embodiments, length of edge 2412 maybe substantially equal to or slightly larger than the width of air gap2410.

FIG. 24B illustrates a backward-facing fin arrangement. In this example,shaft 140 rotates in a clockwise direction as illustrated at 2406.Moving inward toward the axis of rotation of machine 2400, each finshifts opposite of the direction of rotation, in this case,counterclockwise.

FIG. 25 illustrates a rotating electrical machine with a rotor that hasa plurality of fins, such as fin 2550, for cooling according to someembodiments. Fin 2550 extends outward from the rotor winding into theair gap between the rotor and stator and may conduct heat away from therotor winding. In one embodiment, the rotor may have a single finextending out between each of its cores. The fins circulate air andincrease surface area for heat to dissipate from the rotor. The fins mayhave a height from the rotor less than a width of the air gap.

In some embodiments, heat is dissipated from stators 132A, 132B androtor 134 using a cooling fin assembly 2602. FIG. 26 shows rotor 134with cooling fin assembly 2602. Cooling fin assembly 2602 may include acentral portion 2604 configured to attached to rotor 134. Centralportion 2604 may attach to the outer surface of rotor 134 using aninterference fit to form a fin-cool stator assembly 2600. A plurality offins 2606 extend radially from central portion 2604.

Fins 2606 may be evenly spaced around central portion 2604. Fins 2606may be formed in a variety of shapes and sizes. For example, as shown inFIG. 26, fins 2606 are substantially planar. They widen as they extendin the radial direction from the machine, forming a wider upper finportion 2608 and a narrower lower fin portion 2610. In some embodiments,other features of machine 2400 may dictate the shape of fins 2606. Forexample, fin 2606 may have narrow lower fin portion 2610 to increasespace around rotor 134 for additional components. For example, widerupper fin portion 2608 may enclose stator coils and their connectionbars.

In some embodiments, fins 2606 may be angled. Fins 2606 may be angledsuch that fins 2606 do not extend tangentially from central portion 2604(i.e. are angled either into or away from the direction of rotation ofthe rotor 134). In addition fins 2606 may be angled such that fins 2606do not extend parallel to the central axis of machine 2400. A finconnector 2612 may extend between fins 2606. Fin connector 2612 is anabutment from a ring in compression with the rotor and may provideadditional structural support.

In some embodiments, cooling fin assembly 2602 is formed as a unitarystructure, that is, as a single piece. For example, components ofcooling fin assembly 2602 may be machined, molded, die cast, or 3Dprinted

Components of cooling fin assembly 2602 may be formed from a variety ofmaterials including aluminum, aluminum alloys, titanium and others. Insome embodiments, cooling fin assembly 2602 is formed from materialshaving a high thermal conductivity so heat can effectively be removedfrom the inner components of machine 2400. For example, cooling finassembly 2602 may be formed of materials having a thermal conductivitybetween 100 and 1,000 Wm⁻¹K⁻¹. In some embodiments, the material formingcooling fin assembly may be doped to further increase thermalconductivity. This may increase thermal conductivity beyond 1,000Wm⁻¹K⁻¹.

Fins 2606 increase the surface area of machine 2400 to increase the rateof convection heat transfer. In addition, since fins 2606 are attachedto rotor 134, fins 2606 increase conduction heat transfer away fromrotor 134. As explained above, fins 2606 may be formed with a materialhaving high thermal conductivity to increase the rate of conductionbetween rotor 134 and fins 2606, and the rate of convection between fins2606 and the environment. In operation, fins 2606 rotate with rotor 134.The rotation of fins 2606 drives air through machine 2400. Thisincreases the airflow through machine 2400 and contributes to cooling bybringing cooler air in contact with components of machine 2400,including fins 2606.

In some embodiments, the rotation of fins 2606 may increase the airflowthrough machine 2400 by at least 0.12 grams/second mass airflow. In someembodiments, fins 2606 may increase the airflow through machine 2400 bymore. The addition of fins 2606 may keep the temperature of machine2400, including the rotor, under 170 degrees Celsius.

Cooling fin assembly 2602 may be used in conjunction with other coolingmechanisms. For example, cooling fin assembly 2602 may be used inconjunction with impellers 2402A and 2402B. Using impellers 2402A and2402B with cooling fin assembly 2602 my increase the rate of airflowthrough machine 2400 to further increase cooling.

The foregoing description of the specific embodiments will so fullyreveal the general nature of the invention that others can, by applyingknowledge within the skill of the art, readily modify and/or adapt forvarious applications such specific embodiments, without undueexperimentation, without departing from the general concept of thepresent invention. Therefore, such adaptations and modifications areintended to be within the meaning and range of equivalents of thedisclosed embodiments, based on the teaching and guidance presentedherein. It is to be understood that the phraseology or terminologyherein is for the purpose of description and not of limitation, suchthat the terminology or phraseology of the present specification is tobe interpreted by the skilled artisan in light of the teachings andguidance.

The breadth and scope of the present invention should not be limited byany of the above-described exemplary embodiments, but should be definedonly in accordance with the following claims and their equivalents.Identifiers, such as (a), (b) and (i), (ii) are for ease ofidentification and are not meant to imply an order.

1. A rotor for an axial flux rotating electrical machine, the rotor madeby a process comprising: fabricating a cylindrical rotor winding, thecylindrical rotor winding comprising a plurality of cores, each coresubstantially equidistant from an axis, wherein each of the plurality ofcores is comprised of a compressed ferromagnetic powder; fabricating acircular band sized such that the circular band applies compression tothe cylindrical rotor winding when the circular band and the cylindricalrotor winding are at similar temperatures; applying a thermaldifferential between the circular band and the cylindrical rotorwinding; when the thermal differential is applied, inserting thecylindrical rotor winding into an interior of the circular band; andallowing the thermal differential to dissipate placing the cylindricalrotor winding in compression from the circular band.
 2. The rotor ofclaim 1, wherein the band applies a radial compressive force such thatthe compressive force reduces radial deformation of the rotor at highangular velocities.
 3. The rotor of claim 2, wherein the radialcompressive force increases an allowable cyclical loading of the rotor.4. The rotor of claim 1, wherein the band has portions removed tobalance the rotor when the rotor is spinning.
 5. The rotor of claim 1,wherein the band has appendages to conduct heat from the rotor windingand to dissipate heat from the rotor into a surrounding gas.
 6. Therotor of claim 1, wherein each core of the plurality of cores is not apermanent magnet.
 7. The rotor of claim 1, wherein each core of theplurality of cores is isotropic.
 8. The rotor of claim 1, wherein therotor winding comprises a chromium and copper alloy.
 9. The rotor ofclaim 1, wherein each core of the plurality of cores is formed ofpressed iron particles.
 10. The rotor of claim 9, wherein the pressediron particles comprise magnetic particles coated with an insulatinglayer.
 11. The rotor of claim 10, wherein the insulating layer comprisessilica.
 12. A rotor for an axial flux rotating electrical machine, therotor made by a process comprising: fabricating a cylindrical rotorwinding, the cylindrical rotor winding comprising a plurality of cores;and fabricating a circular band sized such that the circular bandapplies compression to the cylindrical rotor winding.
 13. The rotor ofclaim 12, wherein the band is made of maraging steel.
 14. The rotor ofclaim 12, wherein each core of the plurality of cores has a magneticpermeability of at least
 1. 15. The rotor of claim 12, wherein each coreof the plurality of cores has a magnetic permeability of at least 1.5.16. The rotor of claim 12, wherein each core of the plurality of coreshas a saturation magnetic flux density greater than 1.5 T.
 17. The rotorof claim 12, wherein each core of the plurality of cores has asaturation magnetic flux density greater than 2.0 T.
 18. The rotor ofclaim 12, wherein each core of the plurality of cores has a magneticflux density of at least 1.1 T when the core is subjected to a magneticfield of 4,000 Amps/m.
 19. The rotor of claim 12, wherein the bandapplies between of 80 and 300 megapascals of pressure to the rotorwinding.
 20. The rotor of claim 12, wherein the rotor winding isconfigured to prevent the compressive force from adversely affecting theplurality of cores.